Routing devices within a network, often referred to as routers, maintain routing information that describe available routes through the network. Upon receiving an incoming packet, the routers examine information within the packet and forward the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more defined routing protocols, such as a Border Gateway Protocol (BGP) or an Interior Gateway Protocol (IGP).
Multi-protocol Label Switching (MPLS) is a mechanism used to engineer traffic patterns within Internet Protocol (IP) networks. By using MPLS, a source device can request a path through a network, i.e., a Label Switched Path (LSP). An LSP defines a distinct path through the network to carry MPLS packets from the source device to a destination device. A short label associated with a particular LSP is affixed to packets that travel through the network via the LSP. Routers along the path cooperatively perform MPLS operations to forward the MPLS packets along the established path. LSPs may be used for a variety of traffic engineering purposes including bandwidth management and quality of service (QoS).
A variety of protocols exist for establishing LSPs. For example, one such protocol is the label distribution protocol (LDP). Another type of protocol is a resource reservation protocol, such as the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE). RSVP-TE uses constraint information, such as bandwidth availability, to compute paths and establish LSPs along the paths within a network. RSVP-TE may use bandwidth availability information accumulated by an IGP link-state routing protocol, such as an Intermediate System—Intermediate System (ISIS) protocol or an Open Shortest Path First (OSPF) protocol.
Head-end routers of an LSP are commonly known as ingress routers, while routers at the tail-end of the LSP are commonly known as egress routers. Ingress and egress routers, as well as intermediate or transit routers along the LSP that support MPLS, are referred to generally as label switching routers (LSRs). The ingress router uses routing information, propagated from the egress router, to determine the LSP, to assign labels for the LSP, and to affix a label to each packet. The LSRs use MPLS protocols to receive MPLS label mappings from downstream LSRs and to advertise MPLS label mappings to upstream LSRs. When an LSR receives an MPLS packet from an upstream router, the LSR performs a lookup and swaps the MPLS label according to the information in its forwarding table based on the lookup and forwards the packet to the appropriate downstream LSR. The egress router removes the label from the packet and forwards the packet to its destination in accordance with non-label based packet forwarding techniques.